Drive screw assemblies or drive screws are used in many applications to convert angular or rotary motion, typically from a motor, into controlled linear or rotary motion of a mechanical part or machine element. A drive screw assembly typically includes a drive shaft with a female threaded portion that is coupled to the male threaded portion of a drive screw so that the drive shaft and drive screw are connected by intermeshing threads. The threaded portion of the drive screw is coupled to a screw head. As the drive shaft is rotated by a power source, the threaded portion of the drive screw moves the rest of the drive screw in proportional translation relative to the drive shaft. The screw head is consequently extended or retracted along a linear path relative to the drive shaft.
One application of drive screw assemblies is in electromechanical actuators (EMAs). EMAs convert electrical signals into mechanical displacements, and are widely used for many applications. An EMA is typically driven by an electric motor that receives control signals from an EMA controller. The EMA electric motor has a motor shaft that is mechanically coupled to the drive shaft of the associated drive screw assembly. Alternately, the drive shaft of the EMA can be coupled to a secondary drive component, e.g., a gear, that is driven by the motor shaft. For position monitoring and/or control, a position feedback signal may be provided to the EMA controller. Examples of applications in which EMAs are used include the control or actuation of flight control surfaces in aircraft and the control or actuation of fluid flow control devices for hydraulic and pneumatic applications.
FIG. 1 shows a typical prior art drive screw assembly 100 as used in a typical EMA application. The drive screw assembly 100 includes a drive shaft 102, a drive screw 104, and a screw head 106 within a housing 108. The drive shaft 102 has a drive shaft bore 107 with an inner bore surface 107a. The drive screw 104 includes a male threaded portion 104a that is connected by intermeshing threads to the female threaded portion of the drive shaft 102a. The threaded portion 104a of the drive screw 104 does not rotate in unison with, the drive shaft 102. The screw head 106 has a screw head perimeter 106a that is surrounded by the inner bore surface 107a, as shown. The screw head 106 may also have an intermediary portion 106b, e.g., a screw shank, connected to the screw head 106. The drive shaft 102 is positioned within a housing 114 and held by bearings 110(1)-(2), which allow rotational movement of the drive shaft 102. The drive shaft 102 rotates relative to the drive screw 104, the screw head 106 and relative to the housing 114.
As the drive shaft 102 rotates, e.g., when rotated by an electric motor (not shown), the threaded connection between the drive screw 104 and screw head 106 causes the screw head 106 and screw head perimeter 106a to undergo translational movement along the drive shaft bore 107 as the inner bore surface 107a simultaneously rotates around the screw head perimeter 106a. The translational movement of the screw head 106 can be used to move any desired machine or structural element that is connected, directly or indirectly, to the screw head 106. For example, the screw head 106 may be attached to a drag link 116 that is attached to a crankshaft 126, as shown. A suitable crankshaft 124 may have any desired shape, e.g., number and configuration of crank arms, and anchoring or bearing configuration, e.g., it may be connected with bearings to a support 128. A suitable drag link 116 can be connected to the screw head 106 and the crankshaft 124 by pivot connections, 114, 118, respectively, which may include, for example, pivot pins.
One problem that exists with current prior art drive screws including EMAs, such as 100, is the significant efficiency reduction, and corresponding power loss, due to friction forces acting between the screw head 106 and the adjacent rotating parts, e.g., drive shaft inner bore surface 107a. During operation of the prior art drive screw 100, as the screw head 106 is extended or retracted within the drive shaft bore 107 by the rotating motion of the drive shaft 102, the screw head perimeter 106a slides both in linear motion and rotational motion, with respect to the inner bore surface 107a. The sliding movement of the screw head perimeter 106 against the rotating inner bore surface 107a can produce substantial friction forces, which in turn produce power and efficiency losses. The friction forces on the screw head 106 can be increased, making power loss in the screw assembly 100 worse, when side loads, normal to the drive shaft bore 107, are present at the screw head 106. Side loads can result from the geometry of any associated mechanical linkage attached to the screw head, e.g., the drag link skew angle 120, which as used herein represents the degree of position out of parallel, with respect to the drive shaft bore 107. The drag link skew angle 120 may be influenced by the position and geometry of the drag link 116 and its connection to any other elements, e.g., crank arm 124, and pivot angle 122.
As is evident from the description of prior art drive screw assemblies above, friction losses have a significant impact on the efficiency of drive screws, including EMAs. What are needed therefore are methods and apparatus to reduce friction losses and thereby increase the efficiency of drive screws, including those used in EMAs.